1. Field of the Invention
The invention is a unique automotive hybrid powertrain design that allows highly efficient use of energy generated by an integrated internal or external combustion engine. The field of application is in propulsion systems for motor vehicles.
2. The Prior Art
The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including greenhouse gases such as carbon dioxide. For this reason, there has been a quest for approaches to improve the efficiency of fuel utilization for automotive powertrains. Current powertrains typically average only about 10 to 15% thermal efficiency.
Conventional automotive powertrains result in significant energy loss, make it difficult to effectively control emissions, and offer limited potential to bring about major improvements in automotive fuel economy. Conventional powertrains consist of an internal combustion engine and a simple mechanical transmission having a discrete number of gear ratios. Due to the inefficiencies described below, about 85% to 90% of the fuel energy consumed by such a system is wasted as heat. Only 10%-15% of the energy is available to propel the vehicle, and much of this is dissipated as heat in braking.
Much of the energy loss is due to a poor match between engine power capacity and average power demand. The load placed on the engine at any given instant is directly determined by the total road load at that instant, which varies between extremely high and extremely low load. To meet acceleration requirements, the engine must be many times more powerful than the average power required to propel the vehicle. The efficiency of an internal combustion engine varies significantly with load, being best at higher loads near peak load and worst at low load. Since engine operation experienced in normal driving is nearly always at the low end of the spectrum, the engine must operate at poor efficiency much of the time, even though some conventional engines have peak efficiencies in the 35% to 40% range.
Another major source of energy loss is in braking. In contrast to acceleration which requires delivery of energy to the wheels, braking requires removal of energy from the wheels. Since an internal combustion engine can only produce and not reclaim energy, a conventional powertrain is a one-way energy path. Braking is achieved by a friction braking system, which renders useless the temporarily unneeded kinetic energy of the vehicle by converting it to heat.
The broad variation in speed and load experienced by the engine in a conventional powertrain also makes it difficult to effectively control emissions because it requires the engine to operate at many different conditions of combustion. Operating the engine at more constant speed and load would allow much better optimization of any emission control devices, and the overall more efficient settings of the engine would allow less fuel to be combusted per mile traveled.
Conventional powertrains offer limited potential to bring about improvements in automotive fuel economy except when combined with improvements in aerodynamic drag, weight, and rolling resistance. Such refinements can only offer incremental improvements in efficiency, and can apply equally well with improved powertrains.
Hybrid vehicle systems have been investigated as a means to mitigate the foregoing inefficiencies. A hybrid vehicle system provides a "buffer" between the power required to propel the vehicle and the power produced by the internal combustion engine in order to moderate the variation of power demand experienced by the engine. The buffer also allows regenerative braking because it can receive and store energy from sources other than the engine. The effectiveness of a hybrid vehicle system depends on its ability to operate the engine at peak efficiencies and on the capacity and efficiency of the buffer medium. Typical buffer media include electric batteries, mechanical flywheels and hydraulic accumulators.
To use a hydraulic accumulator as the buffer, a hydraulic pump/motor is integrated into the system. The pump/motor interchangeably acts as a pump or motor. As a pump, the pump/motor uses engine or "braking" power to pump hydraulic fluid to an accumulator where it is pressurized against a volume of gas (e.g., nitrogen). As a motor, the pressurized fluid is released through the pump/motor, producing power.
There are two general classes of hydraulic hybrid vehicle systems. A "series" system routes all of the energy produced by the engine through a fluid power path and so it is the fluid power side that experiences the variable road load. This improves efficiency because the efficiency of the fluid power path is not as sensitive to the power demand variations, and because the engine is thus decoupled from road load, allowing it to operate at peak efficiency or be turned off. Series systems are relatively simple in concept and control, but have less efficiency potential than other systems because all energy must be converted to fluid power and back to mechanical power to propel the vehicle. They also depend on frequent on/off operation of the engine for optimum efficiency. "Parallel" systems split power flow between a direct, almost conventional mechanical drive line and a fluid power path. Thus, some of the energy is spared the conversion to fluid power and back again. The most common context for such systems are in a "launch assist" mode where the hydraulic system serves mainly to store braking energy and to redeliver it to assist in the next vehicle acceleration. The parallel system, because it requires both a conventional and a hydraulic power path to the wheels, tends to be more complex than the series system and more difficult to control for smoothness. Depending on the specific design, both series and parallel systems allow some reduction of engine size but both still tend to require a relatively large engine.
For example, U.S. Pat. No. 4,223,532 (Sep. 23, 1980), issued to Shiber, discloses a hydraulic hybrid transmission system which utilizes two pump/motors and is based on a theory that encourages intermittent engine operation.